1. Chemical Basis of Inheritance
Unit VI
Chemical Basis of Inheritance
Dr. Nabil MTIRAOUI, M.Sc, Ph.D
MLT, FAMS, TU

2. Molecular Genetics
The branch of genetics that deals with hereditary transmission and variation on the molecular level.
Deals with the expression of genes by studying the DNA sequences of chromosomes
The study of the molecular structure of genes, involving DNA and RNA.

3. DNA Structure & Properties
Lecture 11

4. I. DNA’s Discovery & Structure

5. What is a Genetic “Factor”?
From Mendel:
we now accepted that there was genetic transmission of traits.
Traits are transmitted by “factors”
Organisms carry 2 copies of each “factor”
The question now was: what is the factor that carries the genetic information?

6. Requirements of Genetic Material
Must be able to replicate, so it is reproduced in each cell of a growing organism.
Must be able to control expression of traits
Traits are determined by the proteins that act within us
Proteins are determined by their sequences
Therefore, the genetic material must be able to encode the sequence of proteins
It must be able to change in a controlled way, to allow variation, adaptation, thus survival in a changing environment.

7. Chromosomes – The First Clue
First ability to visualize chromosomes in the nucleus came at the turn of the century
construction of increasingly powerful microscopes
the discovery of dyes that selectively colored various components of the cell
Scientists examined cellular nuclei and observed nuclear structures, which they called chromosomes
Observation of these structures suggested their role in genetic transmission

8. Implications Chromosomes behaved like Mendel’s “factors”
Mendel's hereditary factors were either located on the chrs or were the chromosomes themselves.
Proof chromosomes were hereditary factors – 1905:
The first physical trait was linked to the presence of specific chromosomal material
conversely, the absence of that chromosome meant the absence of the particular physical trait.
Discovery of the sex chromosomes
"X" and "Y."
distinguished from other chromosomes and from each other

9. What Carries the Genetic Information?
Chromosomes are about 40% DNA & 60% protein.
Protein is the larger component
Protein molecules are composed of 20 different subunits
DNA molecules are composed of only four
Therefore protein molecules could encode more information, and a greater variety of information
protein had the possibility for much more diversity than in DNA
Therefore, scientists believed that the protein in chromosomes must carry the genetic information

10. 1. A History of DNA

11. F. Griffiths (1928)
Tried to determine what genetic material was made of.

12. The Transforming Principle
Fredrick Griffith
Discovered that different strains of the bacterium Strepotococcus pneumonae had different effects on mice
One strain could kill an injected mouse (virulent)
Another strain had no effect (avirulent)
When the virulent strain was heat-killed and injected into mice, there was no effect.
But when a heat-killed virulent strain was co-injected with the avirulent strain, the mice died.
Concluded that some factor in the heat killed bacteria was transforming the living avirulent to virulent?
What was the transforming principle and was this the genetic material?

13. Griffiths’ Experiment

14. Griffiths’ Experiment
Pneumococcus bacteria on mice
2 STRAINS
S-type
Smooth colonies
Virulent
R-type
Rough colonies
Avirulent
Innoculate into mice
Dead from pneumonia
Not killed

15. Avery, MacCleod & McCarthy (1944)
Tried purifying the transforming principle to change R-type Pneumococcus to S-type

16. The Transforming Principle is DNA
Avery, Macleod, & McCarty – 1943
Attempted to identify Griffith’s “transforming principle”
Separated the dead virulent cells into fractions
The protein fraction
DNA fraction
Co-injected them with the avirulent strain.
When co-injected with protein fraction, the mice lived
with the DNA fraction, the mice died
Result was IGNORED
Most scientists believed protein was the genetic material.
They discounted this result and said that there must have been some protein in the fraction that conferred virulence.

17. The Hershey-Chase Experiment
Performed the definitive experiment that showed that DNA was the genetic material.
Bacteriphages = viruses that infect bacteria
Bacteriphage is composed only of protein & DNA
Inject their genetic material into the host

18. The Hershey-Chase Experiment

19. The Experiment Prepared 2 cultures of bacteriophages
Radiolabeled sulphur in one culture
there is sulphur in proteins, in the amino acids methionine and cysteine
there is no sulphur in DNA
Radiolabeled phosphorous in the second culture
there is phosphorous in the phosphate backbone of DNA
none in any of the amino acids.
So this one culture in which only the phage protein was labeled, and one culture in which only the phage DNA was labeled.

20. Experiment Summary
Performed side by side experiments with separate phage cultures in which either the protein capsule was labeled with radioactive sulfur or the DNA core was labeled with radioactive phosphorus.
The radioactively labeled phages were allowed to infect bacteria.
Agitation in a blender dislodged phage particles from bacterial cells.
Centrifugation pelleted cells, separating them from the phage particles left in the supernatant.

21. Results Summary
Radioactive sulfur was found predominantly in the supernatant
Radioactive phosphorus was found predominantly in the cell fraction, from which a new generation of infective phage was generated.
Thus, it was shown that the genetic material that encoded the growth of a new generation of phage was in the phosphorous- containing DNA.

22. Chargaff’s Rule
Chargaff’s rule is a rule about DNA,

23. Chargaff’s Rule
Once DNA was recognized as the genetic material, scientists began investigating its mechanism and structure.
Erwin Chargaff – 1950
discovered the % content of the 4 nucleotides was the same in all tissues of the same species
percentages could vary from species to species.
He also found that in all animals (Chargaff’s rule):
%G = %C %A = %T
This suggested that the structure of the DNA was specific and conserved in each organism.
The significance of these results was initially overlooked

24. The Double Helix: Watson & Crick
Watson and Crick shared the 1962 Nobel Prize for Physiology and Medicine with Maurice Wilkins. Rosalind Franklin died before this date.

25. The Double Helix: Watson & Crick
James Watson and Francis Crick – 1953
Presented a model of the structure of DNA.
It was already known from chemical studies that DNA was a polymer of nucleotide (sugar, base and phosphate) units.
X-ray crystallographic data obtained by Rosalind Franklin, combined with the previous results from Chargaff and others, were fitted together by Watson and Crick into the double helix model.

26. 2. Chemical Bases in DNA
Two types of nucleic acid can be recognized: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
DNA is mostly found in the nucleus where it forms the principal substance of the chromosomal material, the chromatin. In addition to DNA, chromatin contains proteins, mainly histones, and little RNA.

27. 2. Chemical Bases in DNA
In prokaryotes, DNA is present in a single chromosome in the nucleoid.
Little DNA is also found in mitochondria and in chloroplasts.
Many viruses are made up of DNA, mostly double stranded, but some are single stranded.

28. 3. Primary Structure: Nucleotide & Nucleoside
The addition of a pentose sugar to a base produces a nucleoside .
If the sugar is ribose, a ribonucleoside is produced; if the sugar is 2- deoxyribose, a deoxyribonucleoside is produced
Addition of phosphate group to nucleoside produces nucleoside mono-phosphate (NMP) like AMP or CMP or a nucleotide

29. 3. Primary Structure: Mononucleotide

30. 3. Primary Structure: Nitrogenous Bases
PURINES
1. Adenine (A)
2. Guanine (G)
PYRIMIDINES
3. Thymine (T)
4. Cytosine (C)
A or G
T or C

31. 3. Primary Structure: Dinucleotide

32. 3. Primary Structure: Polynucleotide

33. 3. Primary Structure: Polynucleotide

34. 4. Secondary structure: double helical structure
The 2 strands are twisted about each other, coiled around a common axis, forming a right- handed double helix.
The hydrophilic sugar- phosphate backbone of each chain lies on the outside of the molecule. The hydrophobic nitrogenous bases project inwards from the outer sugar-phosphate framework, perpendicular to the long axis of the helix and are stacked one above the other. The stacking of bases is held by hydrophobic bonds .This helps in holding the helical structure.

35. 4. Secondary structure: double helical structure
The nitrogenous bases of the 2 strands meet each other near the central axis of the helix where they become connected by hydrogen bonds between the amino, or imino, hydrogen and the ketonic oxygen atoms. The hydrogen bonding between the bases helps to hold the 2 strands of the DNA together.
Thymine
Adenine
Cytosine
Guanine

36. 4. Secondary Structure: Chargaff’s Rule
A nitrogen-containing ring structure called a base. The base is attached to the 1' carbon atom of the pentose. In DNA, four different bases are found:
two purines, called adenine (A) and guanine (G)
two pyrimidines, called thymine (T) and cytosine (C)
*A always pairs with T : two hydrogen bonds *C always pairs with G : three hydrogen bonds

37. 4. Secondary Structure: Direction of Strands
The 2 strands of the double helical molecule are antiparallel, i.e., they run in opposite direction; one runs in the 5’ to 3’ direction, while the other runs in the 3’ to 5’ direction.

38. 5. DNA Conformations B conformation (B-DNA): A-DNA: Z-DNA:
The most common form of DNA.
The minor groove and major groove, are of different widths on the outside of DNA.
A-DNA:
Forms under conditions of low salt and low humidity.
There can be transient shifts from B to A form.
Z-DNA:
Consists of alternating purines and pyrimidines
Found infrequently.
Z-DNA is:
long and thin
Left-handed,
Phosphate backbone has a zig-zag appearance.

39. 6. Key Features of a DNA molecule

40. 7. Key Features of a RNA molecule

41. 8. Biochemistry of DNA
DNA is the carrier of genetic information, which is stored in the form of a nucleotide sequence. DNA has 2 important functions: “replication” and “transcription”.
Replication
DNA
Transcription
RNA
Translation
Protein

42. 9. What is Gene ?
The gene, the basic units of inheritance; it is a segment within a very long strand of DNA with specific instruction for the production of one specific protein. Genes located on chromosome on it's place or locus.

43. 9. What is Gene ?
A gene in relation to the double helix structure of DNA and to a chromosome (right). Introns are regions often found in eukaryote genes that are removed in the splicing process (after the DNA is transcribed into RNA): only the exons encode the protein. This diagram labels a region of only 40 or so bases as a gene. In reality most genes are hundreds of times larger.